Patients (age 25-45) with angiographically documented coronary artery disease (CAD), and control patients with rheumatic or congenital heart disease were evaluated by history, physical examination, chest film, routine laboratory studies, lipoprotein profile, and determination of cholesterol ester hydrolytic activity (ACE) in purified lymphocytes by a newly developed fluorometric assay. Seventy-one patients have been studied: 36 patients with angiographically documented coronary artery disease and 35 control subjects with other types of heart disease. Patients with premature CAD had a highly significant (p less than .0005) reduction in ACE when compared to the control group. In addition, CAD patients with low ACE activities had fewer other risk factors for CAD than did those with higher ACE activities. These results, taken in conjuction with the lack of correlation between ACE and lipoprotein levels, are consistent with the concept than ACE activity may be a separate risk factor in some patients with premature CAD.